Untrusted packages

Hi,

I have Trisquel 7.

Software Updater tells me there is updated software available for my computer (security updates and other updates).

I click "Install Now".

It asks me to authenticate. There is a "Details" section where I can read:
Action: org.debian.apt.install-or-remove-packages
Vendor: Apt Daemon

I enter my password. A small window pops up reading:

"Requires installation of untrusted packages

This requires intalling packages from unauthenticated sources."

How is that possible?

I have some ppa for some application.

What should I do?

Thank you!